1. Field of the Invention
This invention relates to an improved method for purifying TiO.sub.2 ore which contains numerous impurities including unacceptable levels of naturally occurring radionuclides (NORS) such as thorium and uranium. The purified ore can be used to make TiO.sub.2 pigment or titanium metal or be used in any other process where a purified TiO.sub.2 ore is required. This invention especially relates to removing impurities from titaniferous ores, leucoxene, rutile, perovskite, sphene, and their derivatives or intermediates such as blow-over in the chloride process.
2. Description of Related Art
Currently, approximately 75 percent of the titanium minerals produced in the world are utilized by the pigments industry to produce TiO.sub.2. In the production of TiO.sub.2 by the chloride process, beneficiated ore is used which generally contains about 55-96% TiO.sub.2. The beneficiation processes are aimed at removing impurities such as alkali metals, alkaline earth metals, rare earth metals, iron, aluminum, silicon, phosphorus, thorium, uranium, chromium, manganese, vanadium, and yttrium. These impurities may be present as oxides, salts, or other complex forms. Especially detrimental to the chloride process are such ores which contain in considerable quantity the impurities of iron, calcium, aluminum, phosphorus, magnesium, barium and strontium, and radionuclides such as thorium and uranium (and their daughters of radioactive decay). For example, phosphorus can cause processing problems in the chloride TiO.sub.2 process, and thorium and uranium may concentrate in the TiO.sub.2 process and present a potential health hazard. Also, the impurities of aluminum, iron, phosphorus, thorium, and uranium are additionally a problem because they are especially resistant to removal by conventional mechanical or chemical means. Finally, alkaline earth metals can impair fluidization in the TiO.sub.2 fluidized bed chlorinator.
Being able to remove such impurities efficiently would be highly desirable because known sources of TiO.sub.2 ore not containing such impurities are becoming increasingly scarce and expensive. Moreover, while other processes to purify TiO.sub.2 ore are known, it appears that they either require additional, more complex or more expensive processing steps or are deficient in one or more benefits as compared to the process of this invention.
For titaniferous, leucoxene, rutile, perovskite, and sphene ores, impurities which are especially important to reduce to acceptable levels are iron, manganese, calcium, and radionuclides such as thorium and uranium. It is important that iron be reduced to acceptable levels (1) because it often is a major impurity which can cause substantial chlorine consumption in the chloride process for producing TiO.sub.2, and (2) it will form iron chlorides in the chloride TiO.sub.2 process, and such iron chlorides can be a disposal problem. It also is important that manganese be reduced to acceptable levels. This is because manganese is a high boiling material which can coalesce and form a hard slag on the interior of the flue exiting the fluidized bed chlorinator, which is the first step of the chloride TiO.sub.2 process. Note that manganese is commonly associated with titaniferous ores such as ilmenite. Finally, it is important that the radionuclides be reduced to acceptable levels because they can present potential health problems.
In purifying titaniferous ore, leucoxene, rutile, perovskite, and sphene, it is also important that the TiO.sub.2 content in the ore be upgraded to a reasonably high level so that output of TiO.sub.2 from the TiO.sub.2 process is optimized and processing problems associated with removing ore impurities from the process are minimized. Therefore, generally, the TiO.sub.2 content in the beneficiated ore should be upgraded to at least 75 percent, preferably to at least 80 percent, and most preferably to at least about 90 percent.
While some prior art processes can remove some of the aforementioned impurities, they typically require prereduction or preoxidation followed by prereduction as an essential step. Prereduction and/or preoxidation is undesirable because it is expensive (due to substantial energy and investment requirements) and tends to make it more difficult to remove radionuclides. It therefore would be desirable to have a beneficiation process which did not require prereduction as an essential step.
The following information is disclosed which may be of interest to this invention:
U.S. Pat. No. 4,176,159 discloses a process for the removal of impurities from rutile, ilmenite, and leucoxene ores. The process requires high temperature calcining, cooling, reducing, cooling, magnetic separation, mineral acid leaching, neutralizing, and washing.
U.S. Pat. No. 4,562,048 discloses the beneficiation of titaniferous ores by leaching with a mineral acid. The temperature used is 120.degree.-150.degree. C., and the pressure used is 10-45 pounds per square inch gauge ("psig"). An essential aspect is the venting of water vapor generated during the leaching process. Prior to leaching, the ore is reduced at about 600.degree.-1100.degree. C.
U.S. Pat. No. 4,321,236 discloses a process for beneficiating titaniferous ore. The process requires preheating the titaniferous ore and a mineral acid prior to the leaching operation. The temperature is maintained at 110.degree.-150.degree. C., and the pressure is maintained at 20-50 psig. For ores containing iron in the ferric state, reductive roasting at about 800.degree.-1100.degree. C. is suggested prior to leaching.
U.S. Pat. No. 4,019,898 discloses the addition of a small amount of sulfuric acid to the leach liquor used to beneficiate ilmenite ore. The temperature used is 100.degree.-150.degree. C., and the pressure used is up to 50 psig. For ores containing iron in the ferric state, the ore is reduced prior to leaching at a temperature of about 700.degree.-1200.degree. C.
U.S. Pat. No. 3,060,002 discloses acid leaching of ilmenite and Sorel slag at temperature of 150.degree.-250.degree. C. Prior to leaching, the ore preferably is roasted oxidatively at about 500.degree.-1000.degree. C.